1. Field of the Invention
The present invention relates to a triplet lens system constructed with glass and plastic lenses. Although not limited thereto, it is particularly suitable to lens systems for being used with an image reading device.
2. Description of the Related Art
An image reading device generally consists of a glass lens system, but the glass lens system has its disadvantage of high manufacturing cost that refrains from number of elements of glass lenses. In lower cost without losing resolution, spherical glass lenses with three elements are widely used. Whereas aberration still exists and cannot be completely removed by only using three-element-spherical-glass lenses due to its limited controllable factors. Moreover, the field angle to an optical system may get larger as the whole system becomes smaller. As the field angle gets larger, it is more difficult to get rid of the aberration. In addition, high resolution is maintained by reducing the aperture size though incurring insufficient lightness of the lens system and lower diffractive limit from smaller size of the aperture. Therefore, the manufacturing cost, the field angle, the resolution, the illumination, and the diffractive limit become trade-off variables in the optical design.
The use of the plastic lenses may be considered as a solution to the above-mentioned problems.
First of all, the plastic lens made by injection molding of an injection machine is considered to decrease the cost, easier to be produced into aspheric surface or diffractive surface. Unlike the spherical surface, the aspheric and diffractive surfaces have more controllable factors than the spherical surface to eliminate the image aberration and chromatic aberration so as to improve the resolution and to uplift the field angle. Moreover, the aperture can be larger so as to resolve the illumination and diffractive limit problems.
Second, the use of plastic lens can create new lens layout that differ with common triplet lens system. Usually, a lens system with three elements is suggested positioning with positive-negative-positive allocation, or called Cook Triplet System. In such lens system, position of lenses has the following three types: the first lens with forwarded convex surface, positive meniscus or bi-convex lens; the second lens with bi-concave surface; the third lens with bi-convex surface.
During the manufacturing of the glass lens, it is not easy to control the centering error. Considering the cost of production, in design of the lens, the curvature of the two sides of the lens cannot be too close. The plastic lens is made of injection so that it is easy to control the centering error of the meniscus lens. At the same time, during development of the plastic mold, the meniscus lens can reach a precision better than that of glass lens, thereby enhancing the resolution. Therefore, the meniscus positive or negative lens can be adapted to co-operate with an aspherical or diffractive surface so as to obtain a greater field angle.
In accordance with one aspect of the present invention, the main objective of this invention is to completely resolve aberration by means of applying more than two plastic lenses in a three-element lens system.
In order to achieve the primary objective, the lens system according to a first embodiment of the present invention uses a positive-negative-positive arrangement. The first lens uses a positive meniscus plastic lens instead of the positive meniscus glass lens in the prior art. The second lens uses a forward concave surface and negative meniscus plastic lens instead of the bi-concave glass lens in the prior art. The third lens uses a forward concave surface and positive meniscus glass lens instead of the bi-convex glass lens in the prior art. At least one aspheric surface is mounted on one of the plastic lenses, and a diffractive surface is mounted on the first lens. The focal length of the lens system and the plastic lenses satisfy the following equation:
0.2 less than |f2/f1| less than 0.7xe2x80x83xe2x80x83[1]
0.2 less than |f2/fs| less than 0.6xe2x80x83xe2x80x83[2]
wherein, f1 is the focal length of the first lens, f2 is the focal length of the second lens, and fs is the focal length of the optical lens system.
The reasons for applying the plastic-plastic-glass arrangement are explained as follows.
In the so-called xe2x80x9cCook Triplet Systemxe2x80x9d, the focus of the third lens is desired to be shorter than that of the first lens. In other words, it is desirable that the refractive rate of the third lens is higher than that of the first lens for facilitating the design. In addition, it is easier to find a glass material with a high refractive index so that the plastic material is to be applied in the first lens.
The first lens is made of plastic material in that the plastic material is easier to be made into an aspheric or a diffractive surface so that it has a greater ability to resolve the aberration. Relatively, in addition to be provided with a bi-concave shape, the second lens may also be designed to have a meniscus shape so as to increase the precision of the plastic lens, thereby enhancing the image quality. Equation [1] and equation [2] are thus derived for resolving the aberration problem. It is desired that the first lens has a higher Abbe-number so that the diffractive surface was designed to be mounted in the first lens for improving the chromatic aberration.
The lens system according to a second embodiment of the present invention is the opposite case of the first embodiment. Because the allocation of the lenses was reversed, the relationship between the focal length of the second and the third lenses, and the relationship between the focal length of the second lens and the whole optical system are changed and should to satisfy the following condition.
0.4 less than |f2/f3| less than 0.8xe2x80x83xe2x80x83[3]
0.2 less than |f2/fs| less than 0.6xe2x80x83xe2x80x83[4]
wherein, f2 is the focal length of the second lens, f3 is the focal length of the third lens, and fs is the focal length of the whole optical system.
The lens system according to a third embodiment of the present invention uses a positive-negative-positive arrangement, and all of the lenses are made of plastic. A diffractive surface is allocated in the second lens, and the aspheric surfaces are allocated in the plastic lenses. The focal length of the second lens and the whole lens system should to satisfy the following equation:
|f2/fs| less than 0.6xe2x80x83xe2x80x83[5]
wherein, f2 is the focal length of the second lens, fs the focal length of the whole optical system.
In the equation [5], the aberration problem can be easily rectified. Because the kinds of the optical plastic material are limited, the diffractive surface is used to reinforce the capability of chromatic aberration reduction.
Further benefits and advantages of the present invention will become apparent after a careful reading of the detailed description with appropriate reference to the accompanying drawings.